Distortion properties of an electronic device or device under test (DUT) may be characterized by various means, including measurement of intermodulation distortion (IMD) products in signals output by the DUT. Measurement of IMD products and other distortion properties require a linear receiver with a wide dynamic range (e.g., the ability to measure large signals and small signals simultaneously and accurately). That is, the receiver should not produce internally-generated distortion, which may mask the distortion properties of the DUT.
Conventionally, in order to minimize distortion, the signals received from the DUT are attenuated to prevent overdriving the receiver. While enabling large signals to be measured more effectively, such signal attenuation generally degrades the noise figure of the receiver, masking small distortion products in the receiver's noise floor. Adding a low-noise amplifier (LNA) to the receiver may decrease the noise figure, but the LNA also generates additional distortion.
Attempts improve the dynamic range of the receiver include selectively attenuating the large signals from the DUT, while passing small distortion products that are to be measured. For example, a filter is placed at the receiver input, such that the filter's amplitude variations over frequency may be compensated for as part of a calibration procedure which may yield accurate amplitude measurements. However, since the frequency response of the filter is fixed, multiple filters are needed to measure signals from the DUT over a range of frequencies. This is common practice when measuring IMD products of electronic switches, which can be −130 dB below the fundamental tones, for example. Using multiple filters is labor-intensive, particularly when testing multiple bands, and the calibration procedure is prone to error due to the number of components involved.
In another example, the stimulus (input) signal to the DUT is sampled, changed in amplitude and phase, and then added destructively with the output of the DUT where it is received by test equipment, as described for example by Hassun et al. in U.S. Pat. No. 6,263,289, which is hereby incorporated by reference. However, this technique requires analog phase shifters, attenuators and lines with line stretchers that must be manually adjusted. Also, the phase-delay of the cancellation path is physically large for some DUTs and/or frequencies, and resolution of step attenuators is limited to several tenths of a decibel (dB).